Five element photographic objective

ABSTRACT

An objective of the expanded Tessar type is disclosed. Five elements form four components of which the first counting from the front is positive and convex toward the front, the second is a bi-concave lens, the third is a positive doublet consisting of a negative lens element and a bi-convex lens element cemented together at their adjoining surfaces, and the fourth is a bi-convex lens. The objective fulfills the following conditions: ##EQU1## wherein f is the focal length of the entire lens system; Ri is the radius of curvature of the i-th surface counting from the front; Di is the axial thickness of the i-th glass or air lens element; and Ni is the refractive index of the material of the i-th lens element.

BACKGROUND OF THE INVENTION

This invention relates to photographic objectives and more particularlyto objectives of standard or quasi-standard field angle.

Triplet objectives of the Tessar type, though widely having used inphotographic devices tend to exhibit large longitudinal deviation froman ideal image field in the sagittal plane. At the same time, theirastigmatism is quite large.

Various proposals for improving the aberrations of the Tessar typeobjectives have been made. One of these proposals, disclosed in U.S.Pat. No. 3,011,401 makes use of an additional lens element of bi-convexform positioned at the rear of the objective. With this Tessar type ofobjective system it is quite difficult to achieve a good compromisebetween the requirements of increasing the back focal length and ofminimizing the various aberrations. Another proposed system is disclosedin Japanese Patent Application Laid-Open No.Sho 51-143326 where theadditional lens element is in the form of a positive meniscus lens.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an objective lenssystem of the expanded Tessar type having a standard or quasi-standardfield angle.

Another object of the invention is to provide the objective lens systemof the type described with a long back focal length suited for use witha single lens reflex camera, while simultaneously minimizing increasesin dimensions of the entire system particularly in the axial direction.

A further object of the invention is to provide an improved objectivelens system of the type described in which the positive supplementallens at the rear of the Tessar type objective is utilized forfacilitating flatness of the image field throughout an included angle ofmore than 50° and in which the image field curvature is corrected to aminimum and the astigmatism reduced by a front group of the Tessar typewith a suitable arrangement and choice of glass materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial sectional view of a first embodiment of an objectivelens system according to the present invention.

FIG. 2 is a similar view showing a second embodiment of the invention.

FIG. 3 is a similar view showing a third embodiment of the invention.

FIGS. 4A to 4C are graphic representations of spherical aberrations forthe d line and g line of the sodium spectrum, astigmatism with thesagittal and meridional fans, and distortion of the lens of FIG. 1,respectively.

FIGS. 5A to 5C are similar representations concerning the lens of FIG.2.

FIGS. 6A to 6C are similar representations concerning the lens of FIG.3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 to 3, the objective lens system of the presentinvention comprises a triplet objective of the Tessar type followed by apositive lens element in bi-convex form. The triplet comprises, fromfront to rear, a positive first lens with the front surface convextoward the front, a second lens of bi-concave configuration and apositive third lens which is a doublet consisting of a negative lenselement and a bi-convex lens element cemented together at theiradjoining surfaces.

The system of the invention fulfills the following requirements:##EQU2## wherein f is the focal length of the entire system; R₈ and R₉are the radii of curvature of the front and rear surfaces of the fifthor supplementary lens element respectively; D₄ is the axial airseparation between the bi-concave lens and the doublet; N₁ and N₂ arethe indices of refraction of glasses from which the first and secondelements are made up; D₅ and D₆ are the axial thicknesses of theelements in the doublet respectively; N₃ and N₄ are the indices ofrefraction of the elements in the doublet; R₆ is the radius of curvatureof the cemented surface; and R₃ is the radius of curvature of the frontsurface of the bi-concave lens element.

The requirement (1) represents the configuration of the surfaces of therearmost positive lens element set forth to assist in bringing the imageplane to a flatness in good balance. When the upper and lower limits ofthe range are violated, the image plane is over- and under-corrected atthe marginal and intermediate zones respectively.

Statement (2) defines the axial air separation between the second andthird components. This plays an important part in controlling haloproduced in the sagittal light beam by the second component orbi-concave lens. When this limit is exceeded, the sagittal halo becomesextremely large.

The requirement (3) represents the difference between the refractiveindices of the positive first and negative second lenses, which, whenbelow 0.08, leads to substantial enlargement of the longitudinaldeviations for the position of the sagittal image points.

Statement (4) defines the axial thickness of the doublet. This mostseriously affects coma and astigmatism. When the upper limit isexceeded, the outward coma becomes extremely large. When the lower limitis exceeded, the substantial astigmatism appears in the picture field.

The requirement (5) represents the refractive power of the adjoiningsurfaces of the lenses of the doublet, and affects field curvature,astigmatism and coma aberration.

When the upper limit is exceeded, a large amount of coma is produced.When the lower limit is exceeded, good correction for field curvatureand astigmatism becomes difficult to achieve.

The requirement (6) serves for minimizing the increase in dimensions ofthe system as the back focal length is increased while simultaneouslyachieving good correction of image field. When the lower limit isexceeded, large flare is produced. When the upper limit is exceeded, itbecomes difficult to correct the image field while keeping the size ofthe system to a minimum.

Three examples of specific objective lens systems each having a largerelative aperture of 1:2.5 with a total field angle of 50°, a back focallength of 0.8 f or more, and a total longitudinal length of less than1.44 f measured from the front to the rear vertex can be constructed inaccordance with the numerical data given in the following tables for theradii of curvature, R, the axial separations, D, between the successivesurfaces along with the indices of refraction, Nd, for the d line of thesodium spectrum and the Abbe numbers, νd, for the various lens elements.

EXAMPLE 1

    ______________________________________                                        Focal length f = 100 F-number: 2.5 2ω = 50°                      R            D       Nd          νd                                        ______________________________________                                        1    66.915      12.98   N.sub.1 = 1.757                                                                         ν.sub.1 = 47.9                          2    -895.23     9.98                                                         3    -53.799     2.63    N.sub.2 = 1.62588                                                                       ν.sub.2 = 35.7                          4    68.239      4.05                                                         5    -93.639     3.94    N.sub.3 = 1.64769                                                                       μ.sub.3 = 33.8                          6    62.153      13.63   N.sub.4 = 1.7725                                                                        ν.sub.4 = 49.6                          7    -50.319     0.44                                                         8    301.84      5.19    N.sub.5 = 1.7859                                                                        ν.sub.5 = 44.2                          9    -301.84                                                                  ______________________________________                                         ΣD = 52.84                                                              b.f = 87.32 (Back Focal Length)                                               R.sub.8 /R.sub.9 = -1                                                         N.sub.1 - N.sub.2 = 0.131                                                     D.sub.5 + D.sub.6 = 17.57?                                                    (N.sub.4 - N.sub.3)/R.sub.6 = 0.0020                                     

    __________________________________________________________________________    3rd-order Aberration Coefficient                                              Spherical                Petzval                                              Aberration Coma   Astigmatism                                                                          Sum    Distortion                                    __________________________________________________________________________    1   0.811937                                                                             0.451674                                                                             0.251262                                                                             0.642166                                                                             0.497006                                      2   1.316481                                                                             -1.143061                                                                            0.992485                                                                             0.047999                                                                             -0.903421                                     3   -6.284365                                                                            2.295920                                                                             -0.838788                                                                            -0.713633                                                                            0.567159                                      4   -1.107707                                                                            -1.019213                                                                            -0.937789                                                                            -0.562622                                                                            -1.380545                                     5   0.010476                                                                             -0.028787                                                                            0.079107                                                                             -0.418680                                                                            0.933144                                      6   0.881258                                                                             0.531371                                                                             0.320400                                                                             0.068576                                                                             0.234540                                      7   3.892209                                                                             -0.580501                                                                            0.086579                                                                             0.863828                                                                             -0.141748                                     8   -0.016084                                                                            0.067833                                                                             -0.286087                                                                            0.145405                                                                             0.593325                                      9   1.096741                                                                             -0.56553                                                                             0.291637                                                                             0.145405                                                                             -0.225368                                     Total                                                                             0.600948                                                                             0.009682                                                                             -0.041194                                                                            0.218445                                                                             0.174094                                      __________________________________________________________________________

EXAMPLE 2

    ______________________________________                                        Focal length f = 100 F-number: 2.5 2ω = 50°                      R             D       Nd          νd                                       ______________________________________                                        1     70.336      13      N.sub.1 = 1.804                                                                         ν.sub.1 = 46.6                         2     -1736.6     10.35                                                       3     -60.        2.67    N.sub.2 = 1.64769                                                                       ν.sub.2 = 33.8                         4     79.774      5.51                                                        5     -79.674     3       N.sub. 3 = 1.68893                                                                      ν.sub.3 = 31.1                         6     100         13.69   N.sub.4 = 1.7725                                                                        ν.sub.4 = 49.6                         7     -50.551     0.44                                                        8     200         5.03    N.sub.5 = 1.8061                                                                        ν.sub.5 = 40.9                         9     -335.99                                                                 ______________________________________                                         ΣD = 53.69                                                              b.f = 87.37                                                                   R.sub.8 /R.sub.9 = -0.595                                                     N.sub.1 - N.sub.2 = 0.156                                                     D.sub.5 + D.sub.6 = 16.69                                                     (N.sub.4 - N.sub.3)/R.sub.6 = 0.0008                                     

EXAMPLE 3

    ______________________________________                                        Focal length f = 100 F-number: 2.5 2ω = 50°                      R             D       Nd          νd                                       ______________________________________                                        1      70.989     12.96   N.sub.1 = 1.735                                                                         ν.sub.1 = 49.8                         2      -2734      12.04                                                       3      -50        2.67    N.sub.2 = 1.6398                                                                        ν.sub.2 = 34.5                         4      67.524     2.87                                                        5      -236.22    4.44    N.sub.3 = 1.62004                                                                       ν.sub.3 = 36.3                         6      53.333     15.56   N.sub.4 = 1.7725                                                                        ν.sub.4 = 49.6                         7      -51.493    0.44                                                        8      488.89     4.2     N.sub.5 = 1.7859                                                                        ν.sub.5 = 44.2                         9      -364.83                                                                ______________________________________                                         ΣD = 55.18                                                              b.f = 87.79                                                                   R.sub.8 /R.sub.9 = -1.34                                                      N.sub.1 - N.sub.2 = 0.095                                                     D.sub.5 + D.sub.6 = 20                                                        (N.sub.4 - N.sub.3)/R.sub.6 = 0.0029                                     

What is claimed is:
 1. An objective lens system comprising, from frontto rear, a positive first lens with the front surface convex toward thefront, a bi-concave second lens, a positive third lens consisting of anegative element and a bi-convex lens element cemented together at theiradjoining surface, and a bi-convex fourth lens, and fulfilling thefollowing requirements: ##EQU3## wherein f is the focal length of theentire lens system; Ri is the radius of curvature of the i-th surfacecounting from the front; Di is the i-th axial thickness or airseparation; and Ni is the index of refraction of the i-th lens element.2. An objective lens system according to claim 1, wherein thecharacteristics of the lens elements and their spatial relationship toeach other are substantially as indicated below:

    ______________________________________                                        Focal length f = 100 F-number: 2.5 2ω = 50°                      R             D       Nd          νd                                       ______________________________________                                        1      66.915     12.98   N.sub.1 = 1.757                                                                         ν.sub.1 = 47.9                         2      -895.23    9.98                                                        3      -53.799    2.63    N.sub.2 = 1.62588                                                                       ν.sub.2 = 35.7                         4      68.239     4.05                                                        5      -93.639    3.94    N.sub.3 = 1.64769                                                                       ν.sub.3 = 33.8                         6      62.153     13.63   N.sub.4 = 1.7725                                                                        ν.sub.4 = 49.6                         7      -50.319    0.44                                                        8      301.84     5.19    N.sub.5 = 1.7859                                                                        ν.sub.5 = 44.2                         9      -301.84                                                                ______________________________________                                    

wherein R represents the radii of curvature of the lens surfaces; Drepresents the axial thicknesses or air separations; Nd represents theindices of refractions of the various lens elements; and νd representsthe Abbe numbers of the lens elements.
 3. An objective lens systemaccording to claim 1, wherein the characteristics of the lens elementsand their spatial relationship to each other are substantially asindicated below:

    ______________________________________                                        Focal length f = 100 F-number: 2.5 2ω = 50°                      R             D       Nd          νd                                       ______________________________________                                        1      70.336     13      N.sub.1 = 1.804                                                                         ν.sub.1 = 46.6                         2      -1736.6    10.35                                                       3      -60.       2.67    N.sub.2 = 1.64769                                                                       ν.sub.2 = 33.8                         4      79.774     5.51                                                        5      -79.674    3       N.sub.3 = 1.68893                                                                       ν.sub.3 = 31.1                         6      100        13.69   N.sub.4 = 1.7725                                                                        ν.sub.4 = 49.6                         7      -50.551    0.44                                                        8      200        5.03    N.sub.5 = 1.8061                                                                        ν.sub.5 = 40.9                         9      -335.99                                                                ______________________________________                                    

wherein R represents the radii of curvature of the lens surfaces; Drepresents the axial thicknesses or air separations; Nd represents theindices of refractions of the various lens elements; and νd representsthe Abbe number of the lens elements.
 4. An objective lens systemaccording to claim 1, wherein the characteristics of the lens elementsand their spatial relationship to each other are substantially asindicated below:

    ______________________________________                                        Focal length f = 100 F-number: 2.5 2ω = 50°                      R             D       Nd          νd                                       ______________________________________                                        1      70.989     12.96   N.sub.1 = 1.735                                                                         ν.sub.1 = 49.8                         2      -2734      12.04                                                       3      -50        2.67    N.sub.2 = 1.6398                                                                        ν.sub.2 = 34.5                         4      67.524     2.87                                                        5      -236.22    4.44    N.sub.3 = 1.62004                                                                       ν.sub.3 = 36.3                         6      53.333     15.56   N.sub.4 = 1.7725                                                                        ν.sub.4 = 49.6                         7      -51.493    0.44                                                        8      488.89     4.2     N.sub.5 = 1.7859                                                                        ν.sub.5 = 44.2                         9      -364.83                                                                ______________________________________                                    

wherein R represents the radii of curvature of the lens surfaces; Drepresents the axial thickness or air separations; Nd represents theindices of refractions of the various lens elements; and νd representsthe Abbe numbers of the lens elements.